1. Field of the Invention
The present invention relates to a thin film transistor, a display device having the thin film transistor at least in a pixel portion, and a method for manufacturing the thin film transistor and the display device.
2. Description of the Related Art
In recent years, technology for forming thin film transistors using a thin semiconductor film (with a thicknesses of from several tens of nanometers to several hundreds of nanometers, approximately) formed over a substrate having an insulating surface has been attracting attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in display devices, in particular, is being pushed.
As a switching element in a display device, a thin film transistor including an amorphous semiconductor film, a thin film transistor including a polycrystalline semiconductor film, or the like is used. As a method for forming a polycrystalline semiconductor film, a technique is known in which a pulsed excimer laser beam is processed into a linear shape with an optical system, and an amorphous silicon film is scanned with the linear beam, thereby being crystallized.
As a switching element in a display device, further, a thin film transistor including a microcrystalline semiconductor film is used (see Reference 1: Japanese Published Patent Application No. H4-242724; and Reference 2: Japanese Published Patent Application No. 2005-49832).